1. Field of the Invention
This invention relates to valves, and more particularly, to a pressure equalizing valve for use in a foam fire fighting system.
2. Description of the Prior Art
Balanced pressure proportioning systems are used in the fire fighting industry to provide a predetermined concentration of fire fighting foam solution to foam distribution hardware in order to control and extinguish fires. The main components of a fire fighting foam system are a balanced pressure proportioner, a diaphragm valve for controlling pressure, a foam concentrate pump or other source of pressurized foam, a foam concentrate supply tank, and a source of water.
The balanced pressure proportioner in the system operates to balance or mix the foam concentrate with the water. The balanced pressure proportioner utilizes a venturi in the water line and an orifice in the foam line. Upon equal pressures of the foam and water, a predetermined mixture of the two occurs.
Consequently, the pressure of the foam in the foam inlet line must equal the pressure in the water line to the proportioner for the predetermined mixture of the two to occur. The foam concentrate pump of the system is designed so that it will supply foam concentrate at a pressure equal to the maximum pressure of the water flow, plus a predetermined safety factor.
The water pressure and flow vary due to water supply variations, as well as different types of foam discharge equipment. The foam pump, on the other hand, supplies the foam concentrate at a constant flow and pressure. Thus, the proportion of foam and water, and hence, the solution concentration, varies along with variations in the water pressure and flow.
A diaphragm valve and by-pass line are known devices used to maintain the foam solution at a predetermined mixture. The known diaphragm valve has a diaphragm disposed within a housing that is connected by a shaft to a double seat globe valve. Foam concentrate is supplied through sensing lines to one side of the diaphragm. Water pressure is supplied to the other side of the diaphragm. When the water pressure is lower than the foam concentrate pressure, the globe valve opens, relieving pressure in the foam concentrate line, and allowing excess concentrate to return to the supply tank. When the water supply pressure is higher than the foam concentrate supply pressure, the globe valve closes to increase the pressure and flow of water to the balanced pressure proportioner.
Diaphragm valves currently used in known foam fire fighting systems possess significant disadvantages. The globe valves in the diaphragm valve, used to regulate the supply of foam and water, are expensive. Since such valves incorporate a flexible diaphragm, the diaphragm must be intact, that is, without holes or other perforations, in order for the valve to function. Thus, when the diaphragm is perforated, the valve will not operate properly. Also, the use of a flexible material for the diaphragm allows the diaphragm to have slightly different surface areas on each side, due to variations in the material, and these variations affect the accuracy of the valve over a large pressure range. Many diaphragm valves include springs. The springs are prone to fatigue and failure. In diaphragm valves with springs, the pressure equalization of the foam line pressure to the water line pressure is not always accurate over the entire pressure range because the springs do not have a constant coefficient of elasticity. In addition, the pressure equalization will tend to change as the springs age.